1. Field of the Invention
The present invention relates to a method and apparatus for coding an image signal, a method and apparatus for decoding an image signal, and a recording medium for storing an image signal. More particularly, the present invention relates to a method and apparatus for coding an image signal, a method and apparatus for decoding an image signal, and a recording medium for storing an image signal, which are suitable to transmit an image signal using a transmission system including various transmission lines such as an analog or digital telephone line or a private data transmission line at various transmission rates, suitable to receive an image signal, and suitable to record and reproduce an image signal onto or from a recording medium such as a magnetic disk or RAM having various storage capacities.
2. Description of the Related Art
As shown in FIG. 21, when substantially the same image is represented by a plurality of image signals having different spatial resolutions (a high resolution image signal and a low resolution image signal), if these image signals are coded into a single bit-stream, then it is possible to display the image on a plurality of display devices having different spatial resolutions (a high resolution display device and a low resolution display device) using the single bit-stream.
It is known in the art that if a high resolution image is predicted from a low resolution image, and the low resolution image and the predicted high resolution image are coded into a single bit-stream, then the coding is performed in a highly efficient fashion than the low resolution image and the high resolution image are directly coded. That is, as shown in FIG. 22, a low resolution image is coded first, and then a high resolution image is predicted from the low resolution image. The difference between the high resolution image and the predicted image is then calculated and the result is coded into a bit stream. As a result, as shown in FIG. 23, the low resolution image can be reproduced by decoding a part of the bit stream (the low resolution image data), and the high resolution image can be reproduced by decoding the whole of the bit stream (the low resolution image and difference image) and combining the decoded data.
The coding process shown in FIG. 22 can be performed using a coding apparatus such as that shown in FIG. 24. With the coding apparatus shown in FIG. 24, a low resolution image and a high resolution image are input separately via a low resolution image input terminal 1 and a high resolution image input terminal 4, respectively, ad these images are coded in a hierarchical fashion. The resultant data is output via a high/low resolution image coded data output terminal 8.
The image data input via the low resolution image input terminal 1 is coded by a low resolution image coder 2 and resultant data is supplied to a multiplexer 7. The data coded by the low resolution image coder 2 is also locally decoded, and the locally decoded image data is supplied to a high resolution image predictor 3. The high resolution image predictor 3 predicts a high resolution image from the locally decoded image. A difference calculator 5 calculates the difference between the high resolution image input via the high resolution image input terminal 4 and the predicted high resolution image. The resultant difference image is then coded by a difference image coder 6 into a difference image coded data. A multiplexer 7 multiplexes the difference image coded data with the low resolution image coded data supplied from the low resolution image coder 2 into a high/low resolution coded data (bit stream). The resultant data is output via a high/low resolution image coded data output terminal 8. The bit stream is transmitted to the decoding apparatus shown in FIG. 25 via a recording medium 9 or a transmission line 10.
The decoding process shown in FIG. 23 can be implemented by the decoding apparatus shown in FIG. 25. The high/low resolution coded data (the coded data (bit stream) output from the high/low resolution image coded data output terminal 8 shown in FIG. 24) is input via the recording medium 9 or the transmission line 10 to the high/low resolution coded data input terminal 11 of the decoding apparatus shown in FIG. 25. The hierarchically coded data of the high/low resolution image is decoded into a low resolution image and high resolution image, and output via a low resolution image output terminal 14 and high resolution image output terminal 18, respectively.
In the decoding apparatus, the hierarchical data of the high/low resolution image (high/low resolution coded data in the form of bit stream) received via the high/low resolution coded data input terminal 11 is demultiplexed by a demultiplexer 15 into low resolution image coded data and difference image coded data. The low resolution image coded data is then decoded by a low resolution image decoder 12. To display the low resolution image, the data of low resolution image is transmitted via the low resolution image output terminal 14 to a low resolution monitor (not shown). On the other hand, the high resolution image is predicted by a high resolution image predictor 13 from the low resolution image, and the predicted image is added via an adder 17 with the difference image decoded from the difference image coded data by a difference image decoder 16 thereby generating high resolution image data. The resultant high resolution image data is transmitted via a high resolution image output terminal 18 to a high resolution monitor (not shown) and displayed thereon.
The hierarchical coding technique described above is suitable to code a hard key signal, which is a bilevel image signal. Herein the key signal refers to such a signal which represents the shape of an object by indicating whether specific areas are inside or outside the object. The hard key signal is used when a particular object is extracted from an image and combined with another background image. The hard key signal refers to a key signal which represents whether a particular point is located inside or outside the object using a bilevel signal (for example 1 and 0). The original image from which the key signal is extracted is called a fill image. In the example shown in FIG. 26, when the fill image includes a human figure and the sun taken outdoors, only the image of the human figure is extracted using a key signal and combined with a background image including mountains thereby generating a new mixed image.
The hard key signal is similar to a facsimile signal in that both are bilevel image signals. Therefore, in conventional techniques, the hard key signal is coded in a direct fashion (not in a hierarchical fashion) using the MMR (Modified Modified Read) technique, the chain coding technique in which coding is performed by representing a boundary line between the inside and outside of an object by a chain of arrows each having a starting point and a direction, or the quad tree technique in which the special case where all leaves are the same is prohibited. However, the hard key signal cannot be coded in a highly efficient fashion by these techniques.
In view of the above, it is an object of the present invention to provide a method and apparatus for coding and decoding an image in a highly efficient fashion.